Wiel M. F. Wauben

Climatologies of NOx and NOy: A comparison of data and models

Abstract Climatologies of tropospheric NOx (NO + NO2) and NOy (total reactive nitrogen: NOx + N03 + 2 × N2O5 + HNO2 + HNO3 + HNO4 + ClONO2 + PAN (peroxyacetylnitrate) + other organic ni trates) have been compiled from data previously published and, in most cases, publicly archived. Emphasis has been on non-urban measurements, including rural and remote ground sites, as well as aircraft data. Although the distribution of data is sparse, a compilation in this manner can begin to provide an understanding of the spatial and temporal distributions of these reactive nitrogen species. The cleanest measurements in the boundary layer are in Alaska, northern Canada and the eastern Pacific, with median NO mixing ratios below 10 pptv, NOx below 50 pptv, and NOy below 300 pptv. The highest NO values (greater than 1 ppbv) were found in eastern North America and Europe, with correspondingly high NOy (∼ 5 ppbv). A significantly narrower range of concentrations is seen in the free troposphere, particularly at 3–6 km, with NO typically about 10 pptv in the boreal summer. NO increases with altitude to ∼ 100 pptv at 9–12 km, whereas NOy does not show a trend with altitude, but varies between 100 and 1000 pptv. Decreasing mixing ratios eastward of the Asian and North American continents are seen in all three species at all altitudes. Model-generated climatologies of NOx and NOy from six chemical transport models are also presented and are compared with observations in the boundary layer and the middle troposphere for summer and winter. These comparisons test our understanding of the chemical and transport processes responsible for these species distributions. Although the model results show differences between them, and disagreement with observations, none are systematically different for all seasons and altitudes. Some of the differences between the observations and model results may likely be attributed to the specific meteorological conditions at the time that measurements were made differing from the model meteorology, which is either climatological flow from GCMs or actual meteorology for an arbitrary year. Differences in emission inventories, and convection and washout schemes in the models will also affect the calculated NOα and NOy distributions.

A 3D chemistry transport model study of changes in atmospheric ozone due to aircraft NOX emissions

Abstract The effect of present day aircraft emissions of nitrogen oxides (NOx = NO + NO2) on atmospheric NOx and ozone concentrations is investigated with the global three-dimensional chemistry transport model CTMK. This model uses 12-hourly meteorological data from the ECMWF analysis and includes parameterizations for subgrid scale processes such as convection. CTMK includes an ozone chemistry module containing the methane and carbon monoxide oxidation chain for the troposphere and lower stratosphere. It is found by using the ANCAT aircraft NOx emission invertory that aviation contributes to 20–100 pptv of the NOx at cruise altitudes in northern mid-latitudes, which corresponds to 30–80 and 20–50% of the background mixing ratios for January and July, respectively. This perturbation in NOx occurs mainly in the North Atlantic Flight Corridor and is transported eastwards. The resulting increase in upper tropospheric ozone is 2–3 ppbv (2%) in January and 5–10 ppbv (3%) in July. The ozone perturbation is almost zonally symmetric and attains maximum values at northern mid-latitudes in January and in the polar region in July. The calculated effect of aircraft emissions is found to be small (i.e. less than 5 pptv for NOx and less than 1 ppbv for 03) in the southern hemisphere. The perturbation of NOx by the aircraft emissions at cruise altitudes in northern mid-latitudes is large compared to the standard deviation. Therefore, it is expected that the effect of aviation on NOx is distinguishable from the contribution from other NOx sources. As the modelled natural variability of ozone is already about 30%, it will not be easy to detect the ozone perturbation due to aircraft NOx emissions.

On the magnitude of transport out of the Antarctic polar vortex

The degree of isolation of the Antarctic stratospheric vortex in late winter and spring is investigated quantitatively by using a three-dimensional global tracer transport model, in which the transport is computed from European Centre for Medium-Range Weather Forecasts analyzed data. The evolution of the spatial distribution of passive tracers provides information about variations in the vortex structure, as well as about the magnitude of the transport out of the Antarctic vortex. The vortex structure revealed by tracers released inside the vortex at 72.5 hPa corresponds well with the satellite-derived distribution of total ozone. The model computations indicate that in late winter and spring of the years 1990-1993, there is a quasi-horizontal cross-vortex transport of about 0.24% per day of the total tracer amount, while per day, 0.83% of the vortex mass descends into the troposphere. This indicates that roughly 65% of the vortex air is flushed out during August-September-October, the approximate lifetime of the Antarctic vortex. This number is insensitive to changes in model resolution, although the quasi-horizontal outflow into the midlatitude stratosphere increases at the expense of the downward outflow if a coarser resolution is used. It is concluded that during late winter and early spring (i.e., the period of major ozone depletion), the vortex is a fairly well isolated air mass.

The effects of the conversion of nitrogen oxides in aircraft exhaust plumes in global models

A parametrization is needed in global models to account for the sub-grid chemical processes taking place in the plume of an aircraft, since these processes can cause the conversion of a considerable amount of the emitted NOx to reservoir species, such as HNO3. For this purpose, the chem- ical conversions of nitrogen oxides in the plume of an air- craft were investigated with a newly developed model. The calculated fractions of dierent nitrogen compounds formed within 24 hours in the exhaust plumes, dierentiated for the global domain and season, were used to modify the original aircraft NOx emissions from the ANCAT emission inven- tory to emissions of various nitrogen compounds and we ap- plied these to the global Chemistry Transport Model KNMI (CTMK). The results obtained imply that neglect of aircraft plume processes in global modeling leads to an overestima- tion of the NOx and O3 perturbations. Compared with a CTMK calculation with unmodied aircraft NOx emissions, the NOx perturbations in the North Atlantic flight corridor (NAFC) decreased by 15%-55%, due to conversions in the plumes. The resulting O3 perturbation decreased by 15%- 25%.

Comparison of modeled ozone distributions with sonde and satellite observations

The global distribution of ozone in the troposphere and lower stratosphere calculated with a three-dimensional chemistry transport model driven by European Centre for Medium-Range Weather Forecasts (ECMWF)-analyzed meteorological fields has been compared with observed ozonesonde profiles. This comparison is presented in a new graphical format, which shows in a single panel the vertical and seasonal dependence. The modeled ozone profiles compare reasonably well with climatological ozonesonde data for various stations all over the world, especially if the variability of the ozone concentrations is taken into account. However, the ozone mixing ratios in the upper troposphere and lower stratosphere at midlatitudes are generally overestimated by the model. This is probably caused by a combination of an overestimation of the stratosphere-troposphere exchange and the absence of heterogeneous reactions in the lower stratosphere which reduce ozone. The latitudinal dependence and seasonal dependence of the observations are reproduced by the model calculations, except for the ozone concentrations at the surface. This might be due to the neglect of nonmethane hydrocarbons, which give rise to photochemical ozone production during summer, although other factors such as emissions and deposition cannot be ruled out. The ozone column density obtained by combining calculated ozone distributions up to 50 hPa with climatological zonal mean data for ozone above 50 hPa compares reasonably well with total ozone mapping spectrometer (TOMS) observations. Particularly, the variability caused by synoptic features observable in modeled total ozone shows a high degree of correspondence to the observations. This indicates that rapid variations in the ozone column density are mainly the result of corresponding variations in ozone concentrations near the tropopause due to transport. Modeled total ozone is generally underestimated in the tropics and overestimated elsewhere compared to the TOMS observations. This overestimation, which is large in spring at northern midlatitudes and increased toward the pole, can partly be ascribed to differences between the prescribed climatological ozone concentrations above 50 hPa and the actual values in 1990. Budget calculations of tropospheric ozone showed that reducing these prescribed ozone concentrations lowers the ozone input from the stratosphere, which is largely compensated by a higher photochemical ozone production in the troposphere such that deposition, which depends on the ozone concentrations in the lower troposphere, remains almost unaffected. The reason for the overestimation needs to be investigated further using in situ measurements of several trace gases simultaneously in order to better understand the chemical processes involved. In this study a methane and carbon monoxide oxidation chemistry scheme has been employed without stratospheric chemistry. Furthermore, the comparison of TOMS total ozone observations in the tropics with model calculations seems to suggest that the treatment of ozone precursors such as the NOx emissions by lightning and biomass burning needs to be improved. The reduced correlation between observed and modeled total ozone fields at southern midlatitudes can probably be ascribed to the lower quality of the analyzed fields in that area. Because of the above mentioned discrepancies the calculated atmospheric impact of anthropogenic emissions needs to be interpreted with care.

The passive transport of NOx emissions from aircraft studied with a hierarchy of models

Abstract The passive transport of aircraft emissions of nitrogen oxides (NOx = NO + NO2) has been studied with a hierarchy of models ranging from two-dimensional and three-dimensional chemistry transport models up to three-dimensional models of the general circulation. The sink of NOx was parameterized by an exponential decay process with a globally constant half-lifetime of 10 days. By performing a simple experiment the importance of the various transport processes has been studied. The three-dimensional models show that the monthly mean volume mixing ratio of NOx varies by a factor of three in the longitudinal direction and the temporal variability is of the order of 30%. In view of the nonlinearity of the chemical processes leading to ozone formation in the presence of NOx this implies that the assessment of the effects of subsonic aircraft emissions of NOx should be done with three-dimensional models. Vertical redistribution by convection strongly affects the maximum NOx mixing ratio at cruise altitudes, but due to the limited lifetime of NOx of the order of ten days the most important contribution to the mixing ratio at a certain level usually stems from emissions around that level. The strong static stability in the stratosphere hampers significant dispersion of the subsonic aircraft emissions above the height where the emissions take place for the lifetimes considered here. Some model deficiencies and biases have been identified and discussed. Examples are the oscillatory signature of NOx distributions obtained with a spectral advection scheme, the strong diffusion of one of the GCMs into the polar regions, and the too intense interhemispheric exchange of one of the two-dimensional CTMs. For the vertical redistribution of the emissions it may be necessary to include not only updrafts but also downdrafts in the convective parametrization of the transport model.

Determination of mixing layer height from ceilometer backscatter profiles

Mixing layer height (MLH) is a key parameter in many atmospheric boundary layer studies and processes. A Wavelet method is developed for the automatic determination of mixing layer height from backscatter profiles of an LD-40 ceilometer. Furthermore, a quality flag is introduced to identify unreliable MLH detections. The performance of the Wavelet MLH algorithm is analysed by comparing the results with MLH estimates from radiosondes, wind profiler and research lidar measurements. A correlation coefficient of 0.64 is found between ceilometer and radiosonde determinations when using only ceilometer MLH detections with good quality. A statistical analysis of the ceilometer MLH for a six year data set shows satisfactory results for availability and the results show the main characteristics of MLH, i.e. the diurnal and seasonal cycle. However, problems arise e.g. in case of multiple (well defined) aerosol layers, which renders the selection of the correct mixing layer top ambiguous. Furthermore, in spring and summer the detection of the MLH for deep (convective) boundary layer often fails. This is mostly due to the high variability of the aerosol backscatter signal with height which limits the range for MLH estimation in those conditions.